The present invention relates to apparatus for plotting graphic information on sheet material and, more particularly, it is concerned with a high accuracy drum plotter having a drum to which a sheet of plotting material is directly and securely attached for precise movement with the drum.
Prior art plotters for producing graphic information on sheet material have been of two general types. A flat bed plotter utilizes a system of two carriages which are suspended over the flat support surface of a stationary table and translate a plotting head mounted on the carriages over the table along corrdinate directions or axes. A drum plotter such as disclosed in U.S. patent application, Ser. No. 353,117, filed Apr. 20, 1973 and owned by the assignee of the present invention has a rotatable plotting drum which moves the plotting material in one coordinate direction about the driven axis and a carriage-mounted plotting head which moves in another coordinate direction parallel to the axis of the drum.
From the standpoint of high speed performance, the drum plotter offers the advantage that the relatively heavy drive motors associated with each of the coordinate directions may remain stationary. In flat bed plotters, the drive motors and associated carriages which translate the plotting head usually move with the head and, hence, the total weight and corresponding inertia of the movable components reduces the speed, acceleration and accuracy with which such plotters operate. The plotting paper, however, is held stationary during plotting and may be drawn securely against the support surface of the flat bed by means of a vacuum system. Of course, drum plotters such as disclosed in the above-identified application present accuracy problems in feeding the plotting material, generally a high quality paper, back and forth between storage rolls at opposite sides of the plotting drum. Eliminating movements of the paper relative to the support surface of the plotting drum in the course of a plotting operation removes the element of error accompanying such movement.
Throughput of information is also an important measure of plotter performance. Throughput is defined as the quantity information or data processed in a given unit of time. In plotters, the factors which limit throughput are the maximum velocities and accelerations of the plotting head and the paper relative to one another, assuming that the electrical components of the data handling equipment can keep up with the mechanical components. While both acceleration and velocity are important, changes in the maximum acceleration of a plotter have a greater effect upon throughput if a nominal maximum velocity is obtainable. Acceleration becomes more significant because most plotting operations are comprised of a series of interconnected line segments of relatively short length. Maximum velocity is seldom achieved along such segments. Therefore, if the maximum velocity of a plotter is increased 50% from 2000 ipm (50 mpm) to 4000 ipm (100 mpm), the net increase in throughput may be not more than 10% depending upon the particular graphic being plotted. An increase of 50% in acceleration, however, could increase throughput three or four times that of a corresponding increase in velocity. Thus, the lower moving inertia inherent in drum plotters and the corresponding higher acceleration provide a better basis for higher throughput and performance.
It is also noted that by utilizing a plotting drum on which the plotting material is fixedly secured during a plotting operation, the feed mechanism and drive sprockets which have been used in the past to move the material over the drum may be eliminated. While elimination of such feed mechanism limits the maximum length of plots that may be produced by the drum plotter, many plots requiring high accuracy can be accommodated on standard drawing sheets that are equal to or smaller than the surface of a drum having, for example, a 16 inch (40 cm) diameter. Securing the plotting material fixedly on the drum also utilizes the inherent column strength of plotting material such as paper to prevent wrinkling and thus ensures that the paper will remain spread in a flat condition on the drum support surface.
It is, accordingly, a general object of the present invention to disclose a high performance drum plotter which is constructed to permit high accuracy plotting to be carried out with high throughput.